docs: Fix incorrect usage description of ctx.save_for_backward #4481

Job	Run time
pytorch_tutorial_build_worker (1, 15, linux.g5.12xlarge.nvidia.gpu)	15m 21s
pytorch_tutorial_build_worker (6, 15, linux.g5.4xlarge.nvidia.gpu)	22m 3s
pytorch_tutorial_build_worker (2, 15, linux.g5.4xlarge.nvidia.gpu)	28m 22s
pytorch_tutorial_build_worker (5, 15, linux.g5.4xlarge.nvidia.gpu)	22m 54s
pytorch_tutorial_build_worker (4, 15, linux.g5.4xlarge.nvidia.gpu)	28m 20s
pytorch_tutorial_build_worker (3, 15, linux.g5.4xlarge.nvidia.gpu)	21m 54s
pytorch_tutorial_build_worker (11, 15, linux.g5.4xlarge.nvidia.gpu)	15m 24s
pytorch_tutorial_build_worker (7, 15, linux.g5.4xlarge.nvidia.gpu)	19m 33s
pytorch_tutorial_build_worker (10, 15, linux.g5.4xlarge.nvidia.gpu)	16m 31s
pytorch_tutorial_build_worker (8, 15, linux.g5.4xlarge.nvidia.gpu)	28m 11s
pytorch_tutorial_build_worker (15, 15, linux.g5.4xlarge.nvidia.gpu)	23m 28s
pytorch_tutorial_build_worker (13, 15, linux.g5.4xlarge.nvidia.gpu)	28m 1s
pytorch_tutorial_build_worker (9, 15, linux.g5.4xlarge.nvidia.gpu)	17m 42s
pytorch_tutorial_build_worker (14, 15, linux.g5.4xlarge.nvidia.gpu)	21m 27s
pytorch_tutorial_build_worker (12, 15, linux.g5.4xlarge.nvidia.gpu)	20m 45s
pytorch_tutorial_build_manager	1s
	5h 29m 57s

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